When electrical power is supplied to one or more loads via conductors, the conductors constituting a power distribution network which may for example comprise a conductive grid or a continuous conductive plane, voltage drops arise as a result of the load current flowing through the resistance of the distribution network. These voltage drops reduce the accuracy with which a desired power supply voltage is supplied to each load. Each voltage drop is proportional to the load current, so that the problem of power supply voltage regulation increases with increasing load current.
There is a continuing trend in integrated circuits, and in electronic equipment generally, towards progressively lower power supply voltages, for example to about 1 volt or less, and towards higher load currents, for example of many amps. Such high load currents increase the problem of power supply voltage regulation discussed above. In addition, in view of the low power supply voltages the voltage drops that occur constitute a relatively higher proportion of the desired power supply voltage, thereby exacerbating this problem so that it becomes much more serious.
It is known to improve power supply voltage regulation by sensing the power supply voltage at a point close to a load and by using the sensed voltage information to adjust a source of the power supply voltage, for example a power supply having a regulated output voltage, so that the voltage at the sensing point is substantially constant. This is referred to as remote sensing, because the sensing point is located at the load, i.e. distant or remote from the output of the power source itself, even though the actual distance may be small.
In the case of many loads distributed spatially along a power distribution bus constituting the distribution network, remote sensing cannot eliminate the problem for all of the loads because the sensing takes place only at one point along the distribution bus. In the case of loads distributed in two or three dimensions, for example several loads constituted by integrated circuits on a printed circuit board (PCB), remote sensing can reduce the problem of power supply voltage regulation to a degree, but it cannot fully compensate at all load points.
This problem increases as power supply voltages decrease and as current increases, as is the present trend for power supply voltages for digital integrated circuits. For example, with progressively higher resolution integrated circuit technology (e.g. 0.35 μm down to about 1 μm and less) power supply voltages are reduced (e.g. from about 3.3 volts to about 1.0 volt or less) in order to avoid problems such as tunneling effects and electrical field breakdown. The lower the power supply voltage, the greater is the relative proportion of a given voltage drop in the power distribution network. Further, such lower power supply voltages may typically be accompanied by corresponding increases in load current, further increasing the voltage drops in the power distribution network.
This problem is further increased in situations where the power distribution network uses conductors with a relatively high resistance. For example this may be the case for distribution of power across the surface of a silicon integrated circuit die, or in a multi-layer PCB in which the thickness of copper layers, including power and ground planes or layers, may be restricted by the number of layers and by minimum trace width requirements. The resistance of the power and ground planes may be further increased by the interruption of these planes by many vias in the PCB.
A known approach to reducing this problem is to use large, low-resistance conductors to distribute power. For example, on a circuit card a plurality of spaced bus bars of substantial size and relatively low resistance may be provided. These bus bars may extend across the card to convey the power supply voltage to multiple points on the card. The bus bars are supplied with the power supply voltage from the power source, for example a regulated voltage power supply on the circuit card, via a further bus bar also of substantial size and positioned at an edge of the card. On a smaller scale, within an integrated circuit, multiple bond wire connections can be provided around the edge of the power plane and, in some cases, to wire-bond to the power plane at interior points of the integrated circuit.
This approach reduces the problem but does not eliminate it, and can introduce other problems due to the size and number of bus bars or bond wires which may be required, and the space that is required for these and their connections.
It is also known, for current sharing purposes and especially to provide redundancy of power supplies in electrical equipment, to provide two power supplies for supplying a power supply voltage to a power supply voltage path. For example, in an equipment rack with a back-plane carrying such a power supply voltage path for connection to a plurality of circuit cards which may be inserted into sockets on the back-plane, a regulated voltage power source may be provided at each side of the equipment rack, i.e. at each end of the power supply voltage path. A voltage sensing point for use in a feedback control loop of the power sources may be provided at a mid-point of the power supply voltage path. In normal operation of such an arrangement load current for other circuit cards connected to the back-plane is shared between the two regulated voltage power sources, and the presence of the two power sources provides redundancy for the power supply. Such an arrangement does not provide a solution for the problem discussed above.